Traditionally, electrical distribution networks use high voltage alternating current infrastructure for the transmission of power. However, as distance increases the inductive and capacitive parasitic losses associated with alternating current become increasingly problematic. In certain applications, for example where power is generated by offshore wind turbines and the power generated must be transmitted onshore by long lengths of subsea cabling, it is more economical to use high voltage direct current power transmission techniques. Typically, this requires the use of power converters to convert the high voltage direct current to alternating current (and vice versa) so that the generated power can be input to the electrical distribution network.
When converting direct current to alternating current it is desirable to maximise power conversion efficiency (i.e. minimise loss of energy occurring during the conversion process) and to minimise harmonic distortion of the output alternating current waveform.
Recently, multilevel power converters have been developed which have high levels of power conversion efficiency and reduced levels of harmonic distortion.
Modular multilevel converters (“MMCs”) are examples of recently developed multilevel power converters. MMCs include a number of energy storage device (“ESD”) arranged in series and controlled to create predetermined voltage levels. Each ESD is connected within the converter using a number of switching valves such that the ESD can either be isolated from, or connected to, the output current path of the converter. By sequentially connecting and isolating different combinations of ESDs, different discrete output voltage levels can be generated. A “stepped” waveform can thus be generated which approximates an alternating current voltage waveform. The greater the number of voltage levels (i.e. the greater the number of steps), the closer the output voltage waveform approximates a sinusoid. The closer the output voltage waveform can approximate a sinusoid, the lower the harmonic distortion. Minimising the harmonic distortion of the alternating current/voltage waveform is important because unwanted higher frequency transients present in the output alternating current/voltage waveform can cause damage to components within the distribution network. Using conventional techniques, the only way to increase the number of voltage levels is to increase the number of modules within the converter. However, whilst reducing harmonic distortion, adding further modules increases the cost of the converter. The additional switching components associated with additional ESDs also introduce additional losses which reduce the overall efficiency of the converter.
It is an aim of certain embodiments of the present invention to mitigate the above-mentioned problems.